Viruses generally are composed of a plurality of different proteins assembled together in a regular arrangement together with DNA or RNA. For viruses that lack a lipid envelope, the proteins are often arranged in a layered or concentric manner with certain proteins cooperating to form the outer coat or capsid and others forming the so-called core or inner capsid.
In many virus species, virus proteins are capable of assembling in the absence of nucleic acid to form so-called virus-like particles or VLPs. Similarly, the proteins which normally cooperate together with nucleic acid to form the virus core can assemble in the absence of nucleic acid to form so-called core-like particles (CLPs). As used herein, the terms "virus-like particles" and "core-like particles" will be used in the above sense to designate assemblages of virus proteins (or modified or chimeric virus proteins) in the absence of virus genomic nucleic acid.
The provision of immunogenic epitopes in entities which are in particulate form is highly desirable as such forms can be especially useful in for example the development of vaccines for oral or other mucosal routes of administration. However, relatively few useful immunogenic epitopes can readily be produced in particulate form wherein the epitopes remain immunogenic. The development of particulate vector systems for the presentation of immunogenic epitopes provides a powerful approach for the delivery of antigens. Various types of particles have been used to present foreign epitopes, including particles formed from the hepatitis B virus (HBV) surface or core antigens, polioviruses and yeast Ty-particles (1-5).
VLPs and CLPs are examples of particulate antigens that possess immnnogenic epitopes. These may often be located at or adjacent to the particle surface, although epitopes may also be located internally. Further, VLPs and CLPs are sufficiently stable and resistant to degradation to enable them to have potential use as vaccines when administered by the oral, respiratory or other mucosal routes, the latter property no doubt being associated with the fact that many viruses in native form are infectious orally. Examples of such viruses are members of the family Reoviridae.
However, attempts to modify native VLPs and CLPs in order to incorporate foreign epitopes have been fraught with difficulties as a result of several factors. First of all, it is often the case that incorporation of foreign proteins of protein fragments into VLPs or CLPs inhibits particle formation. Furthermore, even if particle formation is possible, it is uncertain whether the desired foreign epitope will be immunogenic. For example, it may be located in a site where the foreign epitope is incapable of assuming its natural conformation.
It has recently been reported that when the two major inner capsid proteins (VP3 and VP7) of bluetongue virus (BTV, Orbivirus genus, Reoviridae) are synthesized in insect cells by a dual recombinant baculovirus, viral core-like particles (CLPs) are formed (6). These particles can be isolated by one-step sucrose gradient centrifugation or by salt precipitation.
It has also been found that expression of the two major outer capsid proteins (VP2 and VP5) together with VP3 and VP7 using suitable recombinant baculoviruses results in the synthesis of VLPs. Further, expression of different combinations of minor internal proteins VP1, VP4 and VP6, both with the components of CLPs (VP3 and VP7) or VLPs (VP2, VP5, VP3, VP7), results in the inclusion of VP1, VP4 and/or VP6 into CLPs or VLPs respectively.
Further, the synthesis of CLPs can be obtained by co-expression of VP3 and VP7 species representing different serotypes of BTV or other orbiviruses such as epizootic haemorrhagic disease virus (EHDV) VP3 and BTV VP7. Further the synthesis of VLPs can be obtained with genes representing different BTV serotypes.